I met my wife Mindy on our first day of college. We quickly became friends, and stayed friends for over two years until we started dating our junior year. During our friendship we never did anything sexual, never even kiss.
She was also friends with Andy. He was a slightly heavyset guy who went to high school with Mindy and they had been friends for a while. The three of us quickly became kinda of the "three amigos". We spent a lot of time together, hanging out in each others dorm rooms etc.
There were times that I would have a girlfriend or Mindy would have a boyfriend that we weren't around eachother as much but we were pretty much always together.
Once Mindy and I started dating, Andy pretty much disappeared. When it was time for us to get married Mindy asked me if it was okay that we invite Andy. I didn't see why not, but it seemed strange that she asked me. She couldnt believe that I didn't know so she told me the whole story.
During those two and a half years, Mindy and Andy were fucking like rabbits the whole time. I NEVER knew!! She said that they would leave my dorm room at night and go straight to his and do it. She said it was sometimes 5-6 times a week. They were from the same hometown (I lived an hour away) and they spent the whole summers just going at eachother. She said she never liked him in "that way" and it was just straight up sex.
I still can't believe that I didnt see what was happening, I asked her if she would have done a threesome with us back then and she said no.
Andy came to our wedding and we still keep in facebook touch with him. I have other specific stories about those college years with Mindy, Andy and me if you would like.